1. Field of the Invention
The present invention relates to a laser scanning apparatus used for a digital copier or a laser printer.
2. Description of the Related Art
In a typical laser scanning apparatus used for a digital copier or a laser printer, a bundle of divergent light rays emitted from a semiconductor laser light source is converted by a collimating lens to a bundle of parallel light rays, or a bundle of concentrated light rays. Then, the bundle of light rays is scanned by being reflected by a polygon mirror, and passes through a plurality of optical lenses and mirrors so that an image is formed on a photosensitive body.
Recently, a recording method using a multi-beam laser has been used to promote a high speed of a laser beam printer. For this method, a semiconductor laser (hereinafter, referred to as “LD”), in which a plurality of light emitting points are monolithically formed, is used as the light source. The monolithic multi-beam LD generally has a wide interval between the light emitting points.
In particular, Japanese Patent No. 2554724 (Publication No. 01-250922) discloses that it is difficult to manufacture a visible ray type monolithic multi-beam LD having an interval between light emitting points of less than 70–100 μm.
When a monolithic multi-beam LD having a wide interval between light emitting points is used, many problems are generated, according to the installation method thereof.
FIG. 1 is a view showing an imaginary line drawn through the light emitting points of a semiconductor laser that is perpendicular to a main scanning direction. Referring to FIG. 1, when the semiconductor laser is used such that the imaginary line thereof is perpendicular to a main scanning direction, that is, a direction in which a light beam is scanned by being reflected by a polygon mirror, distortion of an image surface is generated in a scanning lens having a great power in a sub-scanning direction, which is perpendicular to the main scanning direction, so that a high level of optical performance cannot be obtained.
In the optical scanning apparatus, to correct inclination of a polygon mirror surface, the light emitting point of the LD and the polygon mirror surface are formed optically to be a conjugate function in the sub-scanning direction, which is perpendicular to the main scanning direction. Since the light emitted from the light emitting point of the LD is enlarged by 2–10 times on a surface of the polygon mirror, a beam interval on the surface of the polygon mirror is made greater than that on a surface of the LD.
Since the respective light emitting points spaced as above form images on the photosensitive body via a scanning lens, when a scanning lens having a great power in the sub-scanning direction is used, aberration such as image curvature is easily generated.
Also, when a scanning lens having less power in the sub-scanning direction is used, generation of the aberration can be restricted, but a focal length increases so that the size of an optical system increases. Although the same aberration reduction effect can be obtained by decreasing a magnifying power of conjugate between the LD and the polygon mirror, since the efficiency of use of light is lowered, it is not appropriate for a high speed scanning.
Next, FIG. 2 is a view showing a case in which the semiconductor laser is installed with the imaginary line drawn through the light emitting points inclined with respect to the main scanning direction. Referring to FIG. 2, when the LD is installed with the imaginary line inclined with respect to the main scanning direction, since an angle at which a light beam of the LD spreads is greater in a direction perpendicular to the main scanning direction than a direction parallel to the main scanning direction, the beam is vertically lengthy after passing through the collimating lens.
In general, a slit having an opening that is horizontally lengthy is arranged after the collimating lens in the optical scanning apparatus to correct the shape of a beam utilizing a diffraction effect thereof. Thus, the light beam incident on the slit needs to have a sufficiently great strength at the opening of the slit.
However, when the incident beam is vertically lengthy, a sufficiently great strength cannot be obtained at the opening of the slit unless the focal length is increased and the shape of the light beam is magnified. Thus, much of the laser power of the light beam is lost when the light beam passes the slit. Also, when an optical device is provided after the collimating lens to correct the shape of the light beam, the number of parts increases, raising the cost accordingly.